Growth, differential survival, and shifting sex ratio of free-living Libellula pulchella (Odonata: Libellulidae) dragonflies during adult maturation

Abstract

We performed a mark-recapture study to determine rates of change in body mass during maturation of adult free-living Libellula pulchella Drury dragonflies. We captured, weighed, marked and released 444 individuals (278 females, 166 males) that were 0-5d old, including 261 that had emerged on the day of initial capture. On subsequent days, we recaptured 87 individuals at least once and 6 were recaptured more than once. Nearly all new emergents (mean mass = 272 mg) that were recaptured after 1 or 2 d had lost mass (mean change = -29 mg), whereas most individuals recaptured after ≥ 3 days had gained mass (mean change = 109 mg; mean rate = 18mg/d). Individuals that were heavier at emergence were much more likely to gain mass and to be recaptured at ages >1 or 2d, thus suggesting differential survivorship based on size at emergence. Average growth rates of gainers were as high as 57 mg/d over a 5-d period. The sex ratio of newly emerged adult was heavily biased toward females (61%), but males were significantly larger at emergence, were more successful at gaining mass, and the sex ratio of individuals that ultimately gained mass did not differ significantly from 1:1. New emergents had empty guts and minimal fat, and changes in fat and water content explained very little of the average decrease in body mass observed for most new emergents. Overall, our data suggest that L. pulchella dragonflies face severe energy stress during early adult maturation, which strongly affects their demography.